Utility vehicles are small, open vehicles designed for carrying a driver and a passenger in a side-by-side configuration at the front of the vehicle with a load carrying dump box being situated behind them. When used in the ground or turf grooming industry, the vehicle dump box might carry a pile of dirt or sand, or a plurality of rolls of sod, or many other things that are typically used for establishing, grooming or repairing ground or turf surfaces, such as the grass or sand traps found on a golf course. In addition to their basic load carrying and transportation functions, utility vehicles are often used as a platform for directly carrying or for towing various other ground or turf working implements. The Workman® utility vehicles manufactured and sold by The Toro Company, the assignee of this invention, are typical utility vehicles of the type being described here.
Utility vehicles can be converted to sprayers by removing the dump box and by mounting a sprayer in its place. The sprayer includes a tank for holding a liquid, e.g., a liquid fertilizer, herbicide, pesticide or combination thereof, that is to be sprayed atop the ground or turf surface. The tank occupies much of the space that had previously been filled by the dump box. One or more spray booms are carried at the rear of the vehicle with the booms themselves carrying a plurality of laterally spaced, downwardly facing spray nozzles. A spray pump is coupled to a drive shaft of the engine of the vehicle for pumping the liquid out of the tank and through the nozzles on the booms for application to the ground or turf surface as the vehicle is driven over such surface. Since the rotational speed of the spray pump is governed by the rotational speed of the engine, the application rate of the spray liquid is proportional to the rotational speed of the engine.
The use of a belt type continuously variable transmission (CVT) in the drive train of a utility vehicle is documented in US Patent Application Publication 2005/0079937. The use of a CVT provides various advantages over the gear-shift based manual standard transmissions which have been more commonly used in utility vehicles. For one thing, a CVT is an automatic transmission that requires no manual shifting by the operator. Moreover, a CVT is able to provide smooth uninterrupted power without the shift steps and the resulting jerk of standard transmissions.
When a utility vehicle is used as a sprayer as described above, a desired application rate of the sprayed liquid, defined by the amount of spray per unit of area, can be achieved by calibrating and establishing a fixed rate of flow through the spray nozzles relative to the fixed gear ratio of the standard transmission when the utility vehicle is operated in a selected gear of the standard transmission. Then, when the vehicle is driven in the selected gear and since the spray pump and standard transmission are both directly driven by the engine, the application rate is insensitive to vehicle ground speed and will remain substantially constant whether the vehicle is on flat ground, or is going up a hill, or is going down a hill. For example, if going up a hill, the vehicle may slow down through engine lug, but the rotational speed of the spray pump will correspondingly slow down as it is also being driven by the engine. When going down a hill, the vehicle may speed up with the engine speed also increasing, but again the rotational speed of the spray pump will correspondingly increase. In either case, the application rate remains constant.
However, it is the nature of a CVT to provide a continuously changing and varying transmission ratio during operation of the vehicle. For example, when the vehicle is climbing a hill and the vehicle ground speed decreases, a CVT will automatically downshift to a lower transmission ratio. Conversely, when the vehicle is going down a hill and the vehicle ground speed increases, the CVT will automatically upshift to a higher transmission ratio. Transmission ratio is the number of times the output shaft of the CVT revolves for each revolution of the engine, a higher transmission ratio providing more revolutions of the output shaft of the CVT than a lower higher transmission ratio for each revolution of the engine. These constant changes in transmission ratio do not permit one to maintain a constant application rate as the rate of flow through the spray nozzles remains constant despite these variations in the vehicle ground speed.
In order to maintain a substantially constant application rate, one could attempt to create a feedback control loop that would change the rate of flow through the spray nozzles in concert with ground speed. However, this is a relatively expensive and complex control system that is not economically suited for the fairly simple spray operations of the type that would be conducted by utility vehicles. A simpler and more cost effective solution is desirable for allowing a CVT equipped utility vehicle to function as an effective sprayer.